Method of using a germanium layer transfer to Si for photovoltaic applications and heterostructure made thereby

ABSTRACT

Ge/Si and other nonsilicon film heterostructures are formed by hydrogen-induced exfoliation of the Ge film which is wafer bonded to a cheaper substrate, such as Si. A thin, single-crystal layer of Ge is transferred to Si substrate. The bond at the interface of the Ge/Si heterostructures is covalent to ensure good thermal contact, mechanical strength, and to enable the formation of an ohmic contact between the Si substrate and Ge layers. To accomplish this type of bond, hydrophobic wafer bonding is used, because as the invention demonstrates the hydrogen-surface-terminating species that facilitate van der Waals bonding evolves at temperatures above 600° C. into covalent bonding in hydrophobically bound Ge/Si layer transferred systems.

RELATED APPLICATIONS

The present application is related to U.S. Provisional PatentApplication, Ser. No. 60/284,726, filed on Apr. 17, 2001, and claimspriority to it under 35 USC 119.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuantto subcontract number ACQ-1-30619-13 from the NREL.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of semiconductor processing of filmsand in particular to processing nonsilicon films on heterostructures.

2. Description of the Prior Art

Group III-V semiconductor layered structures grown on bulk germaniumsubstrates have been used in the prior art to create high efficiencytriple-junction solar cells with efficiencies greater than 30%. However,these are prohibitively expensive for all but space applications,because the Ge substrate constitutes a large portion of this cost.

Therefore, what is needed is some type of methodology whereby Ge andother nonsilicon films can be made on heterostructures, e.g. integratedwith silicon substrates.

BRIEF SUMMARY OF THE INVENTION

Ge/Si and other nonsilicon film heterostructures are formed byhydrogen-induced exfoliation or spitting of the Ge or nonsilicon filmand then wafer bonded to a cheaper substrate, such a Si, glass, ceramicor the like, as a way to reduce product cost, while, for example,maintaining solar cell device performance from the heterostructuresfabricated from such films. The illustrated embodiment of the inventiondescribes a Ge film, but it is to be expressly understood that othersemiconductor materials could similarly be employed in a mannerconsistent with the spirit and scope of the invention. For example, GaAsand InP films might also be employed.

In the illustrated embodiment by transferring thin, single-crystallayers of Ge to a less expensive Si substrate and reclaiming the donorwafer through a polishing process, a single 300 μm thick Ge wafer servesas a source for transfer of more than one hundred thin Ge layers orfilms.

The bond at the interface of the Ge/Si heterostructures is covalent toensure good thermal contact, mechanical strength, and to enable theformation of an ohmic contact between the Si substrate and Ge layers. Toaccomplish this type of bond hydrophobic wafer bonding is used, becausethe hydrogen-surface-terminating species that facilitate van der Waalsbonding evolves at temperatures above 600° C. into covalent bonding inhydrophobically bound Ge/Si layer transferred systems.

Thus, it can now be understood that one embodiment of the invention isdefined as a method of forming a Ge-based heterostructure comprising thesteps of H⁺ or He⁺ doping a Ge substrate by ion implantation, bondingthe Ge substrate onto a non-Ge substrate and annealing the bondedstructure to exfoliate a Ge film therefrom. Any Ge compatiblecomposition may be used as the substrate with Si being the preferredembodiment. However, InP and GaAs are also expressly contemplatedincluding generally any element or compound which includes elements fromGroups III-V of the periodic table.

The Ge film is bonded onto a Si substrate in the illustrated embodiment.The step of bonding is comprised of the steps of disposing the Ge filmin contact with the non-Ge substrate to define an interface between theGe film and non-Ge substrate; applying at least a first magnitude ofpressure across a corresponding first area of the interface; andannealing the interface under a second magnitude of pressuretherebetween.

The step of applying at least a first magnitude of pressure across theinterface further comprises the steps of sequentially applyingadditional magnitudes of pressure across corresponding areas of theinterface, e.g. the steps of applying sequentially smaller magnitudes ofpressure across sequentially larger areas of the interface. In theillustrated embodiment the step of sequentially applying additionalmagnitudes of pressure comprises the step of sequentially applying threemagnitudes of pressure are applied across three corresponding sizedareas of the interface, namely applying a first magnitude of pressure,which is approximately 24 MPa across an approximately 0.64 cm diameterarea, followed by approximately 6.1 MPa across an approximately 1.3 cmdiameter area, followed by approximately 1.5 MPa across an approximately2.5 cm diameter area. The step of annealing the bonded structure under asecond magnitude of pressure therebetween comprises the step ofannealing the interface at approximately 175° C. under approximately 930kPa of pressure therebetween.

In another embodiment the method further comprises the step ofpassivating the non-Ge substrate prior to disposing the Ge film incontact therewith.

In still another embodiment the method further comprises the step ofdisposing an anti-bubble layer onto the Ge substrate to create ahydrophilic interface therebetween and thus to reduce hydrogen bubbleformation when the Ge substrate is bonded to the non-Ge substrate. Wherethe substrate is Si, the step of disposing a anti-bubble layer onto theGe substrate comprises the step of disposing an amorphous Si layer ontothe Ge substrate to form a Si/a-Si interface by molecular beamdeposition.

In yet another embodiment the method further comprises the steps of wetchemical cleaning the Ge substrate and non-Ge substrate prior to bondingand then annealing the cleaned Ge substrate and non-Ge substrate priorto bonding, namely annealing the cleaned Ge substrate and non-Gesubstrate at approximately 250° C. In N₂.

In another embodiment the method further comprises the step offabricating a semiconductor device onto the Ge-based heterostructure,such as a triple junction solar cell thereon using metal-organicchemical vapor deposition (MOCVD).

In a further embodiment the method further comprises the step ofdisposing a smoothing layer onto the exfoliated Ge film, namely a Gebuffer layer using molecular beam epitaxy. Smoothing techniques that mayalso be applied to the exfoliated film also include CMP and chemicaletching processes.

The invention is also understood to be defined as a Ge-basedheterostructure comprising a Ge film, and a non-Ge substrate bonded tothe Ge film in which the Ge film has been exfoliated from an H⁺ ionimplanted Ge layer by annealing. Again in the illustrated embodiment thenon-Ge substrate is composed of Si, but is expressly meant to includeother elements, compounds, and mixtures which include at least in partelements from Groups III-V of the periodic table.

The Ge substrate and non-Ge substrate are in mutual contact underpressure and annealed to form a covalent bonded interface therebetween.The Ge substrate and non-Ge substrate are brought into mutual contactwith each other and subjected to sequential applications of pressuredistributed over an area of the interface, namely sequentially smallermagnitudes of pressure across sequentially larger areas of theinterface. The non-Ge substrate and Ge substrate are passivated prior tobeing bonded. As discussed above, an anti-bubble layer may be disposedonto the Ge substrate to create a hydrophilic interface therebetween andthus to reduce hydrogen bubble formation when the Ge film is bonded tothe non-Ge substrate, namely where the non-Ge substrate is Si and theanti-bubble layer is amorphous Si. Alternatively, as also discussedabove the non-Ge substrate is rendered hydrophilic or hydrophobic by wetchemical cleaning the Ge substrate and non-Ge substrate prior to bondingand then annealing the cleaned Ge film and non-Ge substrate prior tobonding.

The Ge-based heterostructure is an intermediate structure and generallya semiconductor device will be fabricated on the Ge film. Theillustrated embodiment shows the fabrication of a triple junction solarcell thereon using metal-organic chemical vapor deposition (MOCVD).However, what type of semiconductor device can be fabricated on the Gefilm is quite arbitrary and should be understood to include anysemiconductor device now known or later devised. For example, it isexpressly contemplated that in addition to solar cells that GaAs basedLEDs and lasers will be fabricated on Ge film heterostructures of theinvention

As discussed the Ge-based heterostructure of the invention also mayinclude a smoothing layer onto the exfoliated Ge film, namely a Ge layerdisposed onto the exfoliated Ge film using molecular beam epitaxy.

While the apparatus and method has or will be described for the sake ofgrammatical fluidity with functional explanations, it is to be expresslyunderstood that the claims, unless expressly formulated under 35 USC112, are not to be construed as necessarily limited in any way by theconstruction of “means” or “steps” limitations, but are to be accordedthe full scope of the meaning and equivalents of the definition providedby the claims under the judicial doctrine of equivalents, and in thecase where the claims are expressly formulated under 35 USC 112 are tobe accorded full statutory equivalents under 35 USC 112. The inventioncan be better visualized by turning now to the following drawingswherein like elements are referenced by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a voltage-current curve for a P+Ge/P+Siheterostructure fabricated according to the invention which has beenannealed to 350° C. demonstrate ohmic electric contact.

FIG. 2 a is a vertical cross-sectional view of a scanning electronmicroscope image of a MOCVD triple-junction solar cell structure grownaccording to the invention on a Ge/Se heterostructure template.

FIG. 2 b is a vertical cross-sectional view of a scanning electronmicroscope image of a MOCVD triple-junction solar cell structure grownaccording to the invention on a bulk Ge substrate.

FIG. 3 is a graph of the GaAs band-edge emission photoluminescence ofMOCVD triple-junction tandem solar cells on Ge/Si heterostructures inSamples 1 and 2 shown in dotted line and on bulk Ge in solid line.

FIG. 4 is post-growth RHEED image of a surface showing Bragg rods and areconstructed Ge surface indicating a smooth top plateau.

FIG. 5 a is an atomic force microscopic view of an exfoliated Ge surfaceprior to MBE Ge buffer layer growth indicating a surface roughness ofabout 100 angstrom RMS.

FIG. 5 b is an atomic force microscopic view of the surface of FIG. 5 awhich has been smoothed by the epitaxial growth of a Ge buffer layer toabout 20 angstrom RMS roughness with a mesa geometry.

FIGS. 6 a-6 c are diagrammatic side cross sectional views of the methodof fabrication of one embodiment of the invention.

FIGS. 7 a-7 d are diagrammatic side cross sectional views of the methodof fabrication of another embodiment of the invention.

FIG. 8 is a diagrammatic side cross sectional view of still anotherembodiment of the heterostructure.

The invention and its various embodiments can now be better understoodby turning to the following detailed description of the preferredembodiments which are presented as illustrated examples of the inventiondefined in the claims. It is expressly understood that the invention asdefined by the claims may be broader than the illustrated embodimentsdescribed below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention uses direct wafer bonding in combination withhydrogen-induced layer splitting of germanium to transfer single crystalgermanium (100) films 10 to silicon (100) substrates 12 as shown inFIGS. 2 a and 2 b without using an intervening metallic bonding layer asis typical in the prior art. The metal-free nature of the bond makes thebonded wafers suitable, for example, for subsequent epitaxial growth oftriple-junction GaInP/GaAs/Ge layered solar cells, generally denoted byreference numeral 14, at high temperatures without concern about metalcontamination of the device active region.

The Exfoliation and Rebonding

As shown in FIG. 6 a a germanium crystalline wafer 16 was doped with1×10¹⁷ cm⁻² H⁺ at 80 keV. These wafers 16 are rendered stronglyhydrophobic in the process. The embedded H⁺ coalesce into hydrogen gasbubbles, which induced an upper film 10 of wafer 16 to split off by athermal cycle up to more than 350° C. in an N₂ ambient. Layer splittingis achieved by the formation of hydrogen-containing platelets thatinitiate the propagation of micro-cracks parallel to the Ge surface 18upon annealing to greater than 350° C. with no external pressure onwafer 16. Thus, a film 10 is cleaved or exfoliated from wafer 16 andbonded to wafer 20 as diagrammatically shown in FIG. 6 b. The processcan be repeated many times and wafer 16 repeatedly split to form aplurality of exfoliated films 10. Film thickness can be varied bycontrol of the H⁺ doping concentrations and depths.

For example, in the illustrated embodiment doped germanium crystallinewafer 16 was used to transfer approximately 700 nm thick films 10 withan area of approximately 1 cm². Ge substrate surface 18 is passivated byremoval of any organic contamination by cleaning by acetone and methanolfollowed by a 1 minute deionized water rinse and a 10 second dip in 5%HF acid followed in turn by surface blow drying to remove any oxide fromsurface 18 which is left coated with uncontaminated adsorbed hydrophobicH⁺. Substrate 16 can now be bonded to a different and less costlysubstrate 20 having the desired electrical or physical properties, e.g.a silicon wafer 20 as shown in FIG. 6 c, and annealed to transfer thefilm 10.

Si (100) wafer 20 was passivated by the same wet process sequencedescribed above and a subsequent rinse followed by a deionized waterrinse and a 30 second 80° C. 1:1:3 H₂O₂:NH₄OH:H₂O (RCA1) cleaningprocess followed by a deionized water rinse and a brief HF acid dip toremove the grown oxide. The RCA1 cleaning is included to further reducethe organic surface contamination and remove particles. Followingsurface passivation both Si substrate 20 and Ge substrate 10, they havean RMS roughness well under 0.5 nm as measured on a 5 μm by 5 μm atomicforce microscopy scan.

After passivation substrate 20 and substrate 16 are brought intosurface-to-surface contact with each other as shown in FIG. 6 c andbonding is initiated by a 3500 psi pressure (24 MPa) applied over a0.25-inch (0.64 cm) diameter region at the center of the wafer. Thecontact region is then propagated outward using subsequent pressures of890 psi (6.1 MPa) applied over a 0.5-inch (1.3 cm) diameter region and220 psi (1.5 MPa) applied over a 1.0-inch (2.5 cm) diameter region. Athermal annealing process to 175° C. with an applied pressure of 135 psi(930 kPa) in a modified Parr Instruments pressure cell is used tostrengthen the bonding.

Hydrophobic surface passivation of film surface 18 and a less than 1 nmRMS roughness of surface 18 as measured by contact mode atomic forcemicroscopy along with more than 3000 psi pressure initiation aresuitable conditions for reversible room temperature bonding of substrate16 to substrate 20 to occur. The low strength room temperature bond isthen annealed at 175° C. under 135 psi pressure strengthen the bondingbetween substrate 16 and substrate 20. Annealing the bonded structurewith or without pressure up to >300 C causes film 10 to transfer tosubstrate 20. The covalently bonded heterostructures have been shown tobe stable during thermal cycling from room temperature to 750° C.

Ge-to-Si direct wafer bonding and layer transfer has been achieved asdescribed above, but initial efforts were complicated by gas bubbleformation at the bonded Ge/Si interface 36 between film 10 and substrate20. These bubbles were likely caused by residual interface contaminationpresent at the time of bonding, either in the form of adsorbed water ororganic contamination which subsequently evolved as gas trapped at theinterface 36. These bubbles have been eliminated by two methods.

The first method is by molecular beam deposition of a 40-Å amorphous Silayer 38 on the H-implanted Ge substrate 16 as diagrammatically shown inFIGS. 7 a-7 d. In this case, a hydrophilic or hydrophobic Si—Si waferbond is later formed with substrate 20 using hydrophilic or hydrophobicsurface passivation on both the substrate 20 and the a-Si layer 38deposited on the Ge substrate 16. The room-temperature bonding energyfor hydrophilic Si surfaces is typically about 100 mJ cm⁻² for Si/Sisystems. The Ge/Si heterostructures 14 formed by Si/a-Si hydrophilicbonding show a strong reduction in the total number of interfacialbubbles. Bubble reduction is thought to be due to the increasedhydrophilic bond strength at the Si/a-Si interface verses thehydrophobic room-temperature bond strength of the Ge/Si interface in thepreviously mentioned Ge/Si hydrophobic bonding technique. The higherbond strength increases the bubble pressure required to separate thebonded surfaces and to deform the thin Ge transferred film 10.Additionally, improved organic removal is made possible by the RCA1cleaning of the a-Si layer 38.

The second method to eliminate bubbles at the interface 36 is to use a250° C. pre-bonding anneal in N₂ following wet chemical cleaning, butprior to bonding the hydrophobic Ge and Si surfaces in the methodologyof FIGS. 6 a-6 c. This pre-bonding anneal is thought to desorb water andevolve organic contaminants, leaving a more perfectly H-terminatedsurface. This reduces the bubble defect density in transferred films 10.

The Ohmic Contact

Electrical measurements indicated ohmic I-V characteristics forgermanium layers bonded to silicon substrates with less than 35 ohmsresistance at the interface such as shown in FIG. 1 and described below.

The interface electrical properties were measured by defining Al on aGe/Si⁻ heterostructure 14, prepared by a pre-bonding anneal in N₂ asdescribed above, followed by a layer split anneal at 350° C. The Gesubstrate was Ga-doped to 5×10¹⁷ cm⁻³ and the Si substrate was B dopedto 1×10¹⁸ cm⁻³ in an effort to minimize the junction depletion widthformed at the heterojunction interface 36.

During initial application of a 10 V bias, the Ge/Si interface 36exhibited dielectric breakdown followed by ohmic I-V characteristics insubsequent scans as graphically depicted in FIG. 1. These measurementsindicate an interfacial resistance of 35-40 Ω over a total interfacialarea of about 0.1 cm² for a specific interfacial resistance of about 3.5Ω cm⁻². The Al contact and substrate resistances were determined to benegligible for overall structure resistance. The relatively highinterface resistance is attributed to the fact that the bonded Ge/Sisample was annealed at a maximum temperature of 350° C., lower than thetemperature required for covalent bond formation, which is 600° C. orgreater in Si/Si interfaces.

The Triple Junction Solar Cell

Triple-junction solar cell structures grown on these Ge/Siheterostructure templates by metal-organic chemical vacation deposition,MOVCD, as shown in FIGS. 2 a and 2 b and described below show comparablephotoluminescence intensity and minority carrier lifetime to a controlstructure grown on bulk Ge as depicted in FIG. 3.

Metal organic chemical vapor deposition (MOCVD) growth oftriple-junction solar cell heterostructures 14 in FIG. 2 a on bondedGe/Si substrate 22 was performed using (CH₃)₃Ga and AsH₃ precursors forGaAs cell growth and (CH₃)₃Ga, (CH₃)₃In and PH₃ precursors for GaInPcell growth. The peak temperature during growth was 750° C. and thestructure as shown in FIG. 2 a is comprised of a GaAs buffer layer 24followed by two active base regions, a GaAs base 24 and a GaInP base 28separated by a tunnel junction structure 30.

Photoluminescence (PL) intensity and time-resolved photoluminescence(TRPL) minority carrier lifetime of the structure of FIG. 2 a weremeasured in the heavily doped GaAs top contact layer 32 in a controlsample grown on bulk Ge 34 in FIG. 2 b and structures grown on a Ge/Siheterostructure 14 in FIG. 2 a. Photoluminescence measurements wereperformed with a pump laser operated at λ=457 nm. Because the heavilydoped GaAs contact layer 32 was optically thick to the pump laser,photoluminescence was not observed in the GaInP base 28 or the GaAs baseregion 26, both of which are expected to exhibit higher lifetime andsuperior material quality to the heavily doped GaAs contact layer 32.Time-resolved photoluminescence measurements were performed at NREL witha 600-nm pump laser operated at a repetition rate of 1000 kHz. Thesamples were maintained at 293° K during the measurement. The results ofthe measurement are shown and described in connection with FIG. 3.

Triple-junction solar cell structures 14 as shown in FIG. 2 a were grownby metal-organic chemical vapor deposition (MOCVD) on Ge/Siheterostructures fabricated by hydrophobic wafer bonding. Two of theseGe/Si heterostructures were used as templates for growth and labeledSample 1 and Sample 2, while a control solar cell structure was alsogrown on bulk Ge in the same process as shown in FIG. 2 b. The RMSsurface roughness was measured by contact-mode atomic force microscopy,with the results shown in Table 1.

TABLE 1 Ge roughness in Å Ge roughness in Å Sample Pre-MOCVD Post-MOCVDBulk Ge <5 147 Sample 1 236 897 Sample 2 225 204

Sample 1 exhibited RMS surface roughness four-fold greater than that ofthe GaAs contact layer of Sample 2. These GaAs contact layer roughnessvalues are uncorrelated to the exfoliated Ge surface roughness which wasmeasured, a phenomenon that is not understood at present.Cross-sectional scanning electron micrographs of Sample 1 and the bulkGe control structure are shown in FIGS. 2 a and 2 b respectively. Theseimages show the layer structure of the triple-junction solar cell andthe morphology of the interfaces of the various layers and abruptinterfaces within the microscope resolution, about 100 nm. Sample 1exhibits a rough interface between the layers of the cell structure,with a maximum interface roughness of 0.3 mm located at the GaAs/GaInPinterface 30. Photoluminescence studies of the top GaAs contact layer 32indicate comparable GaAs band-edge emission at 880 nm for the bulk Gecontrol and Sample 2, the smoother epitaxial structure on Ge/Si, asindicated in the graph of FIG. 3. Sample 1 exhibits considerably lowerphotoluminescence intensity than Sample 2. Photoluminescencemeasurements demonstrate an inverse relationship between the GaAscontact layer 32 surface roughness and GaAs contact layer band-edgephotoluminescence intensity, suggesting an increased defect density inthe samples with rougher GaAs contact surfaces. Time resolvedphotoluminescence measurements of the GaAs contact layer 32 indicateshort but comparable decay time constants of τ=0.23 ns for the bulk Gesample and τ=0.20 ns for Sample 2, indicating comparable minoritycarrier life-times in the two structures, if similar surfacerecombination velocities are assumed. The GaAs contact is notpassivated, thus shortening the minority carrier lifetime of the GaAscontact layer 32, due to a high recombination velocity at the exposedsurface. Additionally, the heavy doping in the GaAs contact layer 32also limits the minority carrier lifetime in this layer.

Ge Surface Smoothing

Contact mode atomic force microscopy images of the transferred germaniumsurface generated by the formation of micro-bubbles and micro-cracksalong the hydrogen-induced layer-splitting interface reveals minimum RMSroughness of between 10 nm and 23 nm.

The use of a molecular beam epitaxy Ge buffer layer to smooth thecleaved surface of the Ge heterostructure as shown in FIGS. 5 a and 5 bhas been shown to smooth the surface from about 11 nm to as low as 1.5nm with a mesa-like morphology that has a top surface roughness of under1.0 nm giving a promising surface for improved solar cell growth onsolar cell structures.

The triple-junction solar cell optical performance results indicate thatwithout any surface preparation following the H-induced cleavage of theGe layer 10, high quality Group III-V photovoltaic materials can begrown with good photoluminescence intensity and minority carrierlifetime properties relative to a cell grown on a bulk Ge substrate 34.However, to further improve the optical and electrical properties, it isdesirable to reduce the exfoliated surface roughness. To smooth theexfoliated Ge surface 18 a 250 nm-thick Ge buffer layer 40 as shown inFIG. 8 was grown on the surface of the Ge/Si heterostructure 14 bymolecular beam epitaxy at 450° C. at a rate of 0.1 nm/s. The surfaceevolution was monitored in situ with reflection high electron energydiffraction. The reflection high energy electron diffractionspectrometer (RHEED) pattern as shown in FIG. 4 following the growthalso indicated a smooth (2×1) reconstructed Ge (100) surface. EpitaxialGe growth reduced the surface RMS roughness of the transferred Ge layerfrom about 11 to about 1.5 nm. In addition, the morphology of thesurface drastically changed to a mesa-like form, with a large relativelysmooth layer of less than 1 nm surface roughness, as illustrated in thecomparison of the atomic force micrographs of FIGS. 5 a and 5 b.

In summary, it can now be appreciated that fabrication of high quality,large, e.g. about 1 cm² area Ge (100)/Si (100) heterostructures byhydrophobic wafer bonding and H-induced layer splitting is enabled bythe above disclosure. Bonded Ge/Si heterostructures 14 exhibit ohmicinterfaces and are suitable as templates for heterostructured devices,such as MOCVD growth of InGaP/GaAs/Ge triple-junction solar cellstructures with photoluminescence intensity and decay lifetimescomparable to those found in solar cell structures grown on bulk Ge(100) substrates. Epitaxial growth of Ge buffer layers on transferredGe/Si layers shows promise as a means of reducing the Ge surfaceroughness and improving the optical quality of epitaxial GaInP/GaAs/Gelayers.

Many alterations and modifications may be made by those having ordinaryskill in the art without departing from the spirit and scope of theinvention. Therefore, it must be understood that the illustratedembodiment has been set forth only for the purposes of example and thatit should not be taken as limiting the invention as defined by thefollowing claims. For example, notwithstanding the fact that theelements of a claim are set forth below in a certain combination, itmust be expressly understood that the invention includes othercombinations of fewer, more or different elements, which are disclosedin above even when not initially claimed in such combinations.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim. Although elements maybe described above as acting in certain combinations and even initiallyclaimed as such, it is to be expressly understood that one or moreelements from a claimed combination can in some cases be excised fromthe combination and that the claimed combination may be directed to asubcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptionally equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

1. A Ge-based heterostructure comprising: an exfoliated Ge thin film; anon-Ge substrate; a bond formed between the exfoliated Ge thin film andthe non-Ge substrate; and a multijunction solar cell formed on theexfoliated Ge thin film; wherein the exfoliated Ge thin film is formedby ion implantation into bulk Ge, bonding the bulk Ge with the non-Gesubstrate and annealing the bulk Ge to exfoliate the Ge thin film fromthe bulk Ge.
 2. The Ge-based heterostructure of claim 1, wherein thebulk Ge comprises a Ge substrate.
 3. The Ge-based heterostructure ofclaim 1, wherein the multijunction solar cell comprises a triplejunction solar cell.
 4. The Ge-based heterostructure of claim 1, whereinthe multijunction solar cell comprises a GaAs cell and a GaInP cell. 5.The Ge-based heterostructure of claim 1, wherein the exfoliated Ge thinfilm is formed by H⁺ ion implantation into a first side of a bulk Gewafer, contacting the first side of the bulk Ge wafer with the non-Gesubstrate and annealing the bulk Ge wafer to exfoliate the Ge thin filmfrom the bulk Ge wafer.
 6. The Ge-based heterostructure of claim 1,wherein the non-Ge substrate comprises a semiconductor substrate, suchthat an ohmic contact is formed across an interface between the Ge thinfilm and the non-Ge substrate.
 7. The Ge-based heterostructure of claim1 where the bond is formed by hydrophobically passivating the non-Gesubstrate and the implanted bulk Ge prior to the bond being formed andwhere the Ge thin film is not freestanding, but exfoliated only afterthe bond between the implanted bulk Ge and non-Ge substrate is created.8. The Ge-based heterostructure of claim 1 where the non-Ge substrate iscomposed of Si.
 9. The Ge-based heterostructure of claim 8, wherein anohmic contact is formed across an interface between the Ge thin film andthe non-Ge substrate.
 10. The Ge-based heterostructure of claim 1 wherethe bond comprises a covalent bond formed by contacting the bulk Ge andthe non-Ge substrate under pressure and annealing the bulk Ge and thenon-Ge substrate.
 11. The Ge-based heterostructure of claim 10 where thebond is formed when the bulk Ge and non-Ge substrate are in mutualcontact under sequential steps of pressure distributed over an area ofan interface between the bulk Ge and the non-Ge substrate.
 12. TheGe-based heterostructure of claim 11 where the sequential steps ofpressure distributed over an area of the interface comprise sequentiallysmaller magnitudes of pressure across sequentially larger areas of theinterface.
 13. The Ge-based heterostructure of claim 1 furthercomprising an anti-bubble layer located between the non-Ge substrate andthe Ge thin film to create a bonded interface between the non-Gesubstrate and the anti-bubble layer to reduce hydrogen bubble formationwhen the Ge film is bonded to the non-Ge substrate.
 14. The Ge-basedheterostructure of claim 13 where the non-Ge substrate is Si and theanti-bubble layer is amorphous Si.
 15. A Ge-based heterostructurecomprising: a Ge film; and a non-Ge substrate bonded to the Ge filmwherein the Ge film has been exfoliated from H⁺ ion implanted bulk Ge byannealing, where the non-Ge substrate and the bulk Ge are renderedhydrophilic or hydrophobic by wet chemical cleaning the bulk Ge andnon-Ge substrate prior to bonding and then annealing the cleaned bulk Geand non-Ge substrate to bond the bulk Ge and the non-Ge substrate. 16.The Ge-based heterostructure of claim 15 further comprising asemiconductor device fabricated on the Ge film.
 17. The Ge-basedheterostructure of claim 15, where the non-Ge substrate and the bulk Geare rendered hydrophilic by wet chemical cleaning.
 18. The Ge-basedheterostructure of claim 15, where the non-Ge substrate and the bulk Geare rendered hydrophobic by wet chemical cleaning.
 19. The Ge-basedheterostructure of claim 15, wherein the exfoliated Ge thin film isformed by H⁺ ion implantation into a first side of a bulk Ge wafer,contacting the first side of the bulk Ge wafer with the non-Ge substrateand annealing the bulk Ge wafer to exfoliate the Ge thin film from thebulk Ge wafer.
 20. The Ge-based heterostructure of claim 15, wherein thenon-Ge substrate comprises a semiconductor substrate, such that an ohmiccontact is formed across an interface between the Ge thin film and thenon-Ge substrate.
 21. The Ge-based heterostructure of claim 20, whereincovalent bonds are formed between the Ge thin film and the non-Gesubstrate comprising a silicon wafer.
 22. The Ge-based heterostructureof claim 15, further comprising a multijunction solar cell containing aGaAs cell and a GaInP cell.
 23. A Ge-based heterostructure comprising: aGe film; a non-Ge substrate bonded to the Ge film wherein the Ge filmhas been exfoliated from H⁺ ion implanted bulk Ge by annealing; and asemiconductor device fabricated on the Ge film, where the semiconductordevice fabricated on the Ge film comprises a triple junction solar cell.24. The Ge-based heterostructure of claim 23, wherein the solar cell ismade by using MOCVD and the smoothing layer comprises a Ge epitaxiallayer formed on the exfoliated surface of the Ge thin film.
 25. AGe-based heterostructure comprising: a Ge film; and a non-Ge substratebonded to the Ge film wherein the Ge film has been exfoliated from H⁺ion implanted bulk Ge by annealing; and a smoothing layer on theexfoliated Ge film, wherein the smoothing layer is comprised of a Gelayer disposed onto the exfoliated Ge film using molecular beam epitaxy.26. A Ge-based heterostructure comprising: an exfoliated Ge film havingbulk properties of Ge; a non-Ge substrate bonded to the exfoliated Gefilm; and a multijunction solar cell formed on the exfoliated Ge film;wherein the exfoliated Ge film is formed by ion implantation into bulkGe, bonding the bulk Ge with the non-Ge substrate and annealing the bulkGe to exfoliate the Ge film from the bulk Ge.
 27. The Ge-basedheterostructure of claim 26 where the non-Ge substrate comprises asilicon substrate which is indirectly bonded to the exfoliated Ge filmusing an amorphous silicon bonding layer located between the siliconsubstrate and the Ge film.
 28. The Ge-based heterostructure of claim 26where a bonding layer located between the non-Ge substrate and the Gefilm creates a hydrophilic interface with the non-Ge substrate.
 29. TheGe-based heterostructure of claim 26, wherein the multijunction solarcell comprises a triple junction solar cell.
 30. The Ge-basedheterostructure of claim 26, wherein the multijunction solar cellcomprises a GaAs cell and a GaInP cell.
 31. The Ge-based heterostructureof claim 26, wherein the exfoliated Ge film is formed by H⁺ ionimplantation into a first side of a bulk Ge wafer, contacting the firstside of the bulk Ge wafer with the non-Ge substrate and annealing thebulk Ge wafer to exfoliate the Ge film from the bulk Ge wafer.
 32. TheGe-based heterostructure of claim 26, wherein the non-Ge substratecomprises a semiconductor substrate, such that an ohmic contact isformed across an interface between the Ge film and the non-Ge substrate.33. The Ge-based heterostructure of claim 32, wherein covalent bonds areformed between the Ge film and the non-Ge substrate comprising a siliconwafer.
 34. The Ge-based heterostructure of claim 26 further comprisingan anti-bubble layer located between the non-Ge substrate and the Gefilm to create a hydrophilic interface between the anti-bubble layer andthe non-Ge substrate.
 35. A Ge-based heterostructure comprising: anexfoliated Ge film having the bulk properties of Ge; a non-Ge substratebonded to the exfoliated Ge film; and a smoothing layer on theexfoliated Ge film, wherein the smoothing layer comprises a Ge epitaxiallayer formed on the exfoliated surface of the Ge film.
 36. A method ofmaking a Ge-based heterostructure comprising an exfoliated Ge filmhaving bulk properties of Ge, a non-Ge substrate bonded to theexfoliated Ge film, and a multijunction solar cell formed on theexfoliated Ge film, wherein the method comprises: performing ionimplantation into bulk Ge; bonding the bulk Ge with the non-Gesubstrate, annealing the bulk Ge to exfoliate the Ge film from the ionimplanted bulk Ge; and forming the multijunction solar cell on theexfoliated Ge film bonded to the non-Ge substrate.
 37. The method claim36, wherein the multijunction solar cell comprises a triple junctionsolar cell.
 38. The method of claim 36, wherein: the step of performingion implantation into bulk Ge comprises performing H⁺ ion implantationinto a first side of a bulk Ge wafer; and the step of bonding the bulkGe with the non-Ge substrate comprises contacting the first side of thebulk Ge wafer to the non-Ge substrate.
 39. The method of claim 36,further comprising: hydrophobically passivating the non-Ge substrate andthe implanted bulk Ge substrate prior to the step of bonding; andannealing the non-Ge substrate and the bulk Ge to covalently bond thenon-Ge substrate to the bulk Ge prior to exfoliating the Ge film. 40.The method of claim 36, wherein the non-Ge substrate comprises a siliconsubstrate, such that an ohmic contact is formed across an interfacebetween the Ge film and the non-Ge substrate.
 41. A method of making aGe based heterostructure comprising a Ge film and a non-Ge substratebonded to the Ge film, wherein the method comprises: performing H⁺ ionimplantation into bulk Ge; rendering the bulk Ge and the non-Gesubstrate hydrophobic or hydrophilic by wet chemical cleaning the bulkGe and non-Ge substrate to form a hydrophobically or hydrophilicallypassivated surface on the bulk Ge and a hydrophobically orhydrophilically passivated surface on the non-Ge substrate; contactingthe passivated surface of the bulk Ge with the passivated surface of thenon-Ge substrate; annealing the bulk Ge and the non-Ge substrate to bondthe passivated surface of the bulk Ge to the passivated surface of thenon-Ge substrate; and annealing the bulk Ge to exfoliate the Ge filmfrom the ion implanted bulk Ge.
 42. The method of claim 41, furthercomprising forming a multijunction solar cell on the Ge film.
 43. Themethod of claim 41, wherein: the step of performing ion implantationinto bulk Ge comprises performing H⁺ ion implantation into a first sideof a bulk Ge wafer; the step of bonding bulk Ge with the non-Gesubstrate comprises contacting the first side of the bulk Ge wafer tothe non-Ge substrate; and the step of forming a multijunction solar cellcomprises forming a triple junction solar cell.
 44. The method of claim41, wherein the non-Ge substrate comprises a silicon substrate, suchthat an ohmic contact is formed across an interface between the Ge filmand the non-Ge substrate.
 45. The method of claim 41, wherein the stepof rendering comprises rendering the bulk Ge and the non-Ge substratehydrophobic.
 46. The method of claim 41, wherein the step of renderingcomprises rendering the bulk Ge and the non-Ge substrate hydrophilic.47. A method of making a Ge-based heterostructure comprising a Ge filmand a non-Ge substrate bonded to the Ge film, wherein the methodcomprises: performing ion implantation into bulk Ge; bonding the bulk Geand the non-Ge substrate; annealing the bulk Ge to exfoliate the Ge filmfrom the ion implanted bulk Ge; and forming a smoothing layer on theexfoliated Ge film, wherein the step of forming the smoothing layercomprises epitaxially depositing a Ge layer on the Ge film.
 48. Themethod claim 47, further comprising forming a multijunction solar cellon the smoothing layer.
 49. The method of claim 47, wherein: the step ofperforming ion implantation into bulk Ge comprises performing H⁺ ionimplantation into a first side of a bulk Ge wafer; the step of bondingbulk Ge with the non-Ge substrate comprises contacting the first side ofthe bulk Ge wafer to the non-Ge substrate; and the step of forming amultijunction solar cell comprises forming a triple junction solar cell.50. The method of claim 47, wherein the non-Ge substrate comprises asilicon substrate, such that an ohmic contact is formed across aninterface between the Ge film and the non-Ge substrate.
 51. The methodof claim 47, further comprising: passivating the non-Ge substrate andthe implanted bulk Ge substrate prior to the step of bonding; andannealing the non-Ge substrate and the bulk Ge to covalently bond thenon-Ge substrate to the bulk Ge prior to exfoliating the Ge film.